There is a border across which fertility drops by a factor of two in North America (defined as from Canada to Panama). Specifically, one nation has a TFR of ~4, and the other ~2. Can you guess the two nations? You can find the answer in the charts below.
That’s right, TFR, Guatemala (4.15) → Mexico (2.21) → United States (2.05) → Canada (1.53)
(source, 2005-2010 estimates)
Here’s GDP PPP per capita:
United States ($46,400) → Canada ($38,000) → Mexico ($13,600) → Guatemala ($4,800)
This is why the Mexican-Guatemalan border experiences a great deal of traffic.